1. Field of the Invention
The present invention relates to the technology of controlling the light intensity of illumination devices, and more particularly to a light emitting diode (LED) dimming and driving method of controlling brightness by a current signal and a circuit using the method, and a non pulse width modulation (PWM) signal is used for controlling the brightness to enhance the efficiency of the circuit and the dimming accuracy.
2. Description of the Related Art
In general, a dimming circuits of most LED lamps changes a voltage phase of a power supply by using a switch device such as a silicon controller rectifier (SCR) or a tri-electrode AC switch (TRIAC Switch) and changes the total driving voltage outputted from a driving circuit by switching a phase conduction angle, so as to adjust the driving current inputted into an LED source and achieve a dimming effect. Although such dimming device has the advantages of easy control and simple installation, the voltage waveform of the power supply may be situated at a distortion status to give rise to the problems of a low PF and an increased harmonic voltage. According to the current/voltage (I/V) characteristic curve of LED, LED is a nonlinear element. In other words, the ratio of voltage to current is not a direct proportional ratio, so that the aforementioned dimming method has an inaccurate dimming effect and consumes much power since the change of the driving voltage and the change of driving current are unequal, and thus resulting in an inaccurate dimming effect and consuming much power. Therefore, a conventional driving circuit of an LED generally connects a transistor and a sensing resistor in series and uses the sensing resistor to detect an LED current and then regulates a duty ratio of a pulse width modulation (PWM). By controlling the conduction or disconnection of the transistor by the PWM signal, the outputted driving voltage can be regulated to keep the LED current constant.
Although the aforementioned driving circuit is applicable for the mains power of 80-260V and provides a convenient use, the driving circuit is affected by the properties of the TRIAC component. If the voltage frequency of the PWM signal is too low and the current passing through the TRIAC is lower than the required operating current, the TRIAC will be switched repeatedly, so that the driving current will be not be continuous, and a blinking problem of the LED occurs. On the other hand, if the voltage frequency of the PWM signal is too high, a change of the voltage of the signal will be too quick to cause noises and interferences, result in an abnormal operation of the LED, and reduce practicality. To overcome the aforementioned problem, a conventional PWM driving circuit 1 having a dimmer 10 as shown in FIG. 1 includes a compensation circuit 12 additionally installed next to a control circuit 11 and comprised of a reference voltage generator 120, a voltage sampler 121, a comparator 122 and a control switch 123, wherein the comparator 122 is provided for comparing a reference voltage generated by the reference voltage generator 120 with a sample voltage formed by the voltage sampler 121, and if the sample voltage is smaller than the reference voltage, a control voltage will be outputted to the control switch 123, so that the compensation circuit 12 and the control circuit 11 form a loop, and the control switch 123 provides a hold current to the control circuit 11 to assure the operating stability of the dimmer 10 and prevent the blinking issue of the LED. Although such additionally installed compensation circuit 12 can assure a stable operating quality of the dimmer 10, there is a power loss that affects the overall system efficiency severely, and the method of using the TRIAC to output the voltage signal for the dimming purpose may have a loss of signals during the signal transmission, and thus the dimming efficiency may be affected adversely.
Therefore, it is a main subject of the present invention to improve the dimming mechanism, components and structure of the LED driving circuit to assure the normal operating efficiency of the overall circuit, and the operating stability and the dimming accuracy of the LED lamps.